Add rst-lint doc syntax check

[simondeziel]

ATM, rst-lint turn up many errors in our RST doc:

# rst-lint doc/source/ | cut -d " " -f 1,3- | sort -u
ERROR Unknown directive type "autoclass".
ERROR Unknown directive type "currentmodule".
ERROR Unknown directive type "py:currentmodule".
ERROR Unknown directive type "toctree".
ERROR Unknown interpreted text role "attr".
ERROR Unknown interpreted text role "class".
ERROR Unknown interpreted text role "doc".
ERROR Unknown interpreted text role "func".
ERROR Unknown interpreted text role "py:class".
ERROR Unknown interpreted text role "ref".

Ideally, running tox -e lint should install rst-lint (and its pygments dep) and lint all .rst file there is.